Erosion Control November/December 2016 : Page 29

Installation of box culverts tively long duration of the wet season in the region. Smith says that in the case of the two seasonal wetland plots in question, “the amount of vertical room between the full-service level of the marsh and the upstream property boundary was not that great. By North Dakota rules, we had to keep the top of the embankment below the prop-erty boundary so that, in theory, this embankment would not cause damage or flood land off of the property that we were working on. “Our engineering problem was, ‘how to pass these events?’ Normally, we’d pass them through a primary spill-way, either a sheet-pile structure or a concrete structure, or even a riser and a barrel. And then we’d have a secondary spillway to pass larger events.” Each of those options, however, would have added to the to elevation of the embankment. “At some point, if we had gone with that design philosophy, our marsh would have been virtually nonexistent. We just didn’t have enough elevation difference.” Instead, the engineers decided to situate the spillway in the embankment itself, just one foot below the top eleva-tion. Across the top of the embank-ment, crews cut a depression 18 feet wide and 22 feet long, creating a five-to-one outlet slope on the downstream NOVEMBER/DECEMBER 2016 side. “We placed Flexamat on it. Even though we anticipated low velocities, we were concerned about the long dura-tion of those velocities—the snow melt in the region can be rather lengthy and as we get into spring and summer rains the watershed can just gener-ate some type of flow pretty continuously, so we just didn’t think vegetation alone would support it.” Project construction on the two wetlands began in November, with the danger of winter freeze just weeks, or perhaps days, away. However, as Smith explains, the Novem-ber window is also prime time for working in the Devil’s Lake wetland environment. “Freezing helps our access, of course, but it also hinders our embankment construction process—we don’t want to put any frozen material in that embankment.” On the other hand, he says, fall is “typically our driest time of the year, so it was chosen as the season that we wanted to construct this particular project.” EROSION CONTROL 29